1. Field of the Invention
This invention relates to the use of dynamic light scattering (DLS) for the characterization of the structure and dynamics of such diverse systems as solids, liquid crystals, gels, solutions of biological macromolecules, electrolyte solutions, dispersions of microorganisms, solutions of viruses, membrane vesicles, protoplasm in algae and colloidal dispersions.
2. Brief Description of the Prior Art
Light scattering techniques for particle size characterization follow directly from the work of Mie in 1908 on the scattering of electromagnetic waves from dielectric spheres. The scattering depends upon particle size, wavelength, and refractive index. Techniques for particle sizing based on the angular dependence and polarization of the scattered light intensity have been routinely used to study shapes and sizes of large particles, in the Mie scattering regime. Single particle scattering systems are among some of these techniques utilizing the Mie regime. In the early 1950's it was recognized that the spectrum of the scattered light contains additional hydrodynamic properties of the scatterers, that is, translational and rotational diffusion constants. A time dependent correlation function formalism first developed by Van Hove in 1954 for neutron scattering was extended to light scattering by Komarov and Fisher in 1963. Following the invention of the laser, Pecora in 1964 showed that the frequency of the distribution of light scattered from macromolecular solutions would yield values of the macromolecular diffusion coefficient and, under certain conditions, might be used to study rotational motion and flexing of macromolecules. In 1964, Cummins, Knable and Yeh used an optical mixing technique to spectrally resolve the light scattered from dilute suspensions of polystyrene spheres. Since this pioneering work, applications have proliferated and optical mixing spectroscopy has become a major research field. Optical mixing spectroscopy is concerned with making measurements of the temporal properties of the scattered light in order to study the dynamics of the fluctuations in a fluid.
Measurement of the first order electric field autocorrelation involves elaborate optics and electronics. Direct detection (self-beating) of the scattered light leads to the second order intensity-intensity time correlation function, and homodyne or heterodyne detection leads directly to the first order electric field autocorrelation. In both cases, efficient optical mixing imposes a strict spatial coherence requirement on the optical detection system.
As discussed in U.S. Pat. No. 4,927,268, the process of obtaining particle size information by means of DLS requires relatively sophisticated optics and computer processing. However, this disclosure is not entirely correct in stating that DLS is useful for only small particles of the same size. DLS is routinely used to study highly polydisperse systems, and, also importantly, DLS provides considerably more information regarding the structure and dynamics of the system under investigation. Up until a few years ago, DLS was a specialized tool confined to a research laboratory environment with limited impact on industrial processing. However, simultaneous breakthroughs in the use of semiconductor lasers, miniaturization of the optics, avalanche photodiodes for photon counting, advances in digital electronics, and refinements in data inversion algorithms have opened up a vast area of industrial applications. DLS techniques have one other important advantage over single particle scattering systems as described by U.S. Pat. No. 4,927,268, and that is the large dynamic range over which they can operate. Single particle systems are limited to very dilute suspensions. DLS, however, can probe highly concentrated as well as very dilute systems. In particular, back scatter anemometers are even more effective in very concentrated systems. It is an object of the present invention to provide a generic multiple fiber optic probe which can be adapted to several diverse applications.
The existing state-of-art in fiber optic back scatter anemometers, which combine the transmitted and scattered laser light within the same fiber are those of Dyott [Microwave Opt. And Acoust., 2, 13 (1978)] and of Auweter and Horn [J. Colloid. and Interface Sci., 105, 399 (1985)]. Both types utilize a directional coupler to separate the transmitted and received signals propagating in the same sensor fiber. The transmitted beam emanates into the fluid at the full numerical aperture of the fiber, as defined in the fluid, and thus is not collimated. The back scattered signals are collected over an identical numerical aperture. In addition, the detection process is homodyne, that is, optical radiation reaching the detector comprises the sum of a local oscillator and the scattered signal. For efficient optical mixing, polarization of the two optical fields should be coincident and the wavefronts should be matched. The latter condition is easily satisfied since both optical signals travel in the same monomode fiber. The former condition is more severe and can degrade the optical mixing considerably.
One of the first uses of optical fibers in laser light scattering was described for in situ measurements of blood flow. Dyott [Microwave Optics and Acoust., 2, 13 (1978)] and Ross et al. [J. Colloid and Interface Sci., 64, 545 (1978)] presented a compact back scatter system for applications to particle sizing, motility, and flow measurements. This is believed to be the first portable light scattering apparatus which could be used in the field. It was also the first time that the incident laser beam and received signals were contained in a single unit. Somewhat different configurations have been described by Auweter and Horn [J. Colloid and Interface Sci., 105, 309 (1985)]. Dhadwal and Chu [App. Opt., 28, 411 (1989)] demonstrated the use of an optimized fiber optic receiver for both dynamic and static light scattering. In all the above cases a single fiber has been utilized in the composite probe. Dhadwal and Chu made a compact light scattering spectrometer using many single fiber/GRIN microlens probes.